1. Field of the Invention
The invention relates to passive and active optical device materials. More particularly, the invention relates to polyimide compositions which provide either passive or active wave-guide optical capabilities.
2. Description of the Related Art
Either passive or active wave-guide optical device materials are key components for a wide range of cutting edge optical telecommunication devices. Also, signal processing by optical technology in broadband society will be a key issue to control large amounts of information accurately with short response time. Particularly, there is a growing interest in using active nonlinear optical devices for signal modulation and switching. Additionally, passive optical wave-guide device materials are also crucial components in order to lead optical signals into the active nonlinear optical devices. Organic active non-linear optics materials have several advantages, i.e. large NLO effect, nano- to pico-second response time, and structural design flexibility. Also, these polymer-based materials show better processing ability, mechanical stableness, and cost effectiveness as compared to inorganic crystal materials, such LiNbO3 and BaTiO3. Also, in terms of response time and modulation speed, polymer-based materials have more advantages than inorganic materials, because usually organic polymer-based materials have a lower dielectric constant that leads to faster modulation and switching properties. Also, a passive material is a fundamental material for active optical devices, because this material can be used for a device portion in which optical signals can travel between devices and optical fibers.
Critical requirements for polymer-based optical device materials are high stability (thermal, chemical, photochemical, and mechanical) and low optical loss along with high electro-optic performances.
For achieving high thermal stabilities, high Tg polymer matrix systems are desirable, such as those using polyimide, polyurethane, and polyamide. Particularly, polyimides show excellent thermal stability and are used for various engineering plastics materials. Since polyimide is very stable in terms of chemical, mechanical and temperature properties and possesses excellent optical properties, its major interesting properties for passive or active optical devices include the following:
a. Chemical Stabilities
It is compatible with most microelectronics processes including photolithographic, Reactive Ion Etching (RIE), plasma and sputtering deposition, etc. It has reasonable solvent solubility, and therefore, it can be easily coated as thin film using variety of techniques (spin or spray coatings) before crosslinking.
b. Physical and Thermal Stabilities
Polyimide has a thermal expansion coefficient compatible with silicon, which will be very useful property for integration polymer optical devices with silicon-based microelectronic devices. It is also chemically stable at temperatures as high as 300° C. As recently reported, polyimide type material showed very good thermal stabilities, and no critical deterioration of second-order nonlinear properties was observed at more than 3000 hrs even at 100° C. in the air.
c. Optical Properties
Polyimide has high optical transmission over a wide range from visible to telecommunication wavelengths. In optical wave-guide shape, the transmission loss is reported as lower as 0.1 dB/cm at 1.3 μm.
d. ElectroOptics Properties
When polyimide is loaded with chromophore, it becomes nonlinear polyimide material, and it could have relatively high nonlinearity. A high nonlinear electro-optic coefficient of as high as 35 pm/V has been reported, since matrix polymer and NLO chromophore are usually compatible for a long period of time.
Furthermore, particularly fluorinated polymer have unique features, such as low dielectric constants, low optical loss, and easier workability because of good solvent solubility. Usually, fluorinated polyimide before crosslinking has very good solvent solubility so that it is easily workable for spin-coating processing in fabrication of optical devices.
Also, it is generally known that the more the fluorine atom weight content ratio, the lower the dielectric constant becomes. Usually, lower dielectric constant material can make optical signal traveling speed or modulation speed faster because of less π-electron interaction.
Generally, fluorinated polymer can reduce optical loss of signals. Optical propagation loss includes absorption and scattering losses. Material properties, namely interband electronic absorption of the chromophore and C—H vibration absorption of chromophore and polymer host, contribute to the absorption loss in the polymers. The scattering loss is mainly attributed to dust particles and micro domains introduced during the processing (spin coating, poling, photolithographic processing, and etc.). Therefore, advantages of the fluorinated polymer can contribute mainly to lower the absorption losses. Usually, the wavelengths which are generally used in telecommunication are between 1.3 and 1.5 μm. Thus, if polymer-based materials contain a plenty of C—H bondage, NH2, NH, or OH functional groups in their structures, these moiety vibration absorption in double-frequency area is significant and can give big influence on material absorption.
As reported earlier, polyimide type material showed very good thermal stabilities and no critical deterioration. Sometimes, the second-order nonlinear properties were observed at more than 3000 hrs even at 100° C. in the air. Thus, a combination of polyimide and fluorinated polymer resulted in satisfactory improvement as optical device material. However, sometimes incorporation of chromophore into fluorinated polyimide resulted in lower thermal stabilities. Thus, in order to improve the thermal stabilities, a concept of crosslinking seems to be a practical method to obtain higher and better thermal stableness after crosslinking.
In order to obtain good electrical optical performances, chromophores which are incorporated into host matrix materials are desired for orientation toward the same direction. The chromophores can be orientated to the same direction by polling process or some other proper processes. However, over the time, the direction of chromophores could eventually be disorientated. Particularly, this tendency is observed in the case of low Tg materials. In order to overcome this disadvantage, the concept of crosslinking is very helpful and practical method to obtain higher and better thermal stableness.
As typical crosslinking moieties, epoxy/isocyanate moieties and hydroxyl/amino groups are available. However, after crosslinking, these kinds of moieties result in existence of NH— or —OH group, which contributes to higher absorption in a 1.3 to 1.5 μm wavelength region. On the other hand, as examples of crosslinking moieties which do not result in undesired NH— or —OH group, tri-cyclization of acethylene group, cyanurate ring formation from cynate ester derivatives, difluoro bismaleimide, or trifluorovinyl groups can be crosslinking moiety candidates. However, from standpoints of crosslinking temperature and easiness of synthesis, trifluorovinyl group seems to be the most practical crosslinking moiety, because this group can crosslink around 160-200° C. which are sufficiently lower than the decomposition temperature of thermally unstable other components, such as chromophore.